This work is expected to result in a straight-forward means of quantitatively evaluating the effect on spin label studies of the presence of paramagnetic transition metal ions in the system of interest. In applications of the nitroxyl spin-label technique to biological systems which contain paramagnetic species, interaction of the nitroxyl unpaired electron with other unpaired electrons in the sample can contribute to the line shape of the nitroxyl epr spectrum. Epr studies of systems containing non-equivalent interacting unpaired electrons are rare, and there has been no systematic investigation of the interactions in such systems. It is proposed to investigate in detail under well-defined conditions the interaction of nitroxyl spin-labels with paramagnetic metals. A series of transition metal complexes will be made which incorporate nitroxyl radicals such that the interaction between paramagnetic metal ions and the nitroxyl radical can be studied as a function of the relaxation time of the metal, the distance between the metal and the nitroxyl radical, the viscosity of the medium and the temperature. Computer programs will be written to calculate from first principles spectra for comparison with the experimental spectra.